Who among the following thinkers observed that light and heat are only different forms of the same essential substance and also spoke of gravity as the cause of falling?

Kanad, founder of the Vaisheshika school, held that light and heat are different forms of the same substance and offered an early notion of gravity as the cause of falling.

Nagarjuna was a Buddhist philosopher of the Madhyamaka school known for the doctrine of emptiness, not for physical theories.

Jivaka was a renowned physician of ancient India associated with medicine and surgery.

Ramanuja was a medieval philosopher who propounded the Vishishtadvaita school of Vedanta.

Kanad also developed an early atomic theory, describing matter as composed of indivisible particles called anu.

His ideas appear in the Vaisheshika Sutra, one of the six orthodox systems of Indian philosophy.